I scrapped a 1978 1-ton chevy van based motorhome and took the 35 gallon (or so) tank out of it. I fabbed a few brackets and fitted/hung the tank under the bed, just behind the rear axle. The filler spout is run out the back on the left side, just under the rear bed support. My original tank was trash when I bought my truck and I had the other tank so I made it work. I never liked the idea of a gas tank in the cab.
